Why this happened?

Let me get this straight, you may or may not be responsible for this. If you used your profile to continuously spam Facebook groups, let it be self-promotional posts or any sort of unethical posts, then your profile/post will get reported, and Facebook groups will flag your profile. So, automatically all the posts by your profile will be flagged and won’t be visible to the group members. This is one reason.

If you used your profile to join random Facebook groups and give friend request to the members there, especially the ones in your opposite gender, then this is also one of the possible reasons. It’s not a big deal if they accept your friend request(s), but if they didn’t, and if they report you or decline your requests continuously, then that’s a big deal.

These two are the common reasons, as far as I know, they could have any other reasons as well, I’m not sure.

How you can fix it?

You can try this Facebook Blocks form and let them know about this, trust me, your block might be an accident or misunderstanding as well, so it’s worth a shot!